As of my knowledge this is the first phone that Nokia is releasing only for the developers, please correct me if I am wrong. I am new to this developers only phone what does this means? - it will not fully functioning or still in WIP, is that's the reason for not being public...? Only Nokia internal developers can avails this phones or also the external developers? Is there any change of making this phone sold to all in future?

Also if the developers phone is more advance than N9 (which is public phone) how the code built in N950 can be expected to run in N9 (which is less config than N950)?

As of my knowledge this is the first phone that Nokia is releasing only for the developers, please correct me if I am wrong.

Nokia have a history of providing development devices to external developers. Normally those development devices are prototypes of the final consumer device.

I am new to this developers only phone what does this means? - it will not fully functioning or still in WIP, is that's the reason for not being public...?

The device now known as the N950 was originally intended to be released to consumers under the N9 name near the end of 2010; however, the release was cancelled. So, now a newer device has been given the N9 name, and the unreleased device formerly known an N9 has been dubbed the N950 and made the N9's dev kit.

Only Nokia internal developers can avails this phones or also the external developers?

Nokia are loaning N950s to select external developers.

Is there any change of making this phone sold to all in future?

Nokia have demonstrated that all plans can change, but it seems unlikely this one will.

Also if the developers phone is more advance than N9 (which is public phone) how the code built in N950 can be expected to run in N9 (which is less config than N950)?

Compared the N9 the N950:

has a different screen

has a hardware keyboard

might have a 12 MP camera (some sources say the dev kit has only an 8 MP camera)

lacks NFC

If you like hardware keyboards, the N950 has a clear advantage. If you're developing NFC software or accessories, it is frustratingly inadequate.